Laminography techniques are widely used to produce cross sectional images of selected planes within objects. Conventional laminography requires a coordinated motion of any two of three main components comprising a laminography system, that is, a radiation source, an object being inspected, and a detector. The coordinated motion of the two components can be in any of a variety of patterns including but not limited to: linear, circular, elliptical or random patterns. Regardless of which pattern of coordinated motion is selected, the configuration of the source, object, and detector is such that any point in the object plane is always projected to the same point in the image plane and any point outside the object plane is projected to a plurality of points in the image plane during a cycle of the pattern motion. In this manner, a cross sectional image of the desired plane within the object is formed on the detector. The images of other planes within the object experience movement with respect to the detector thus creating a blur background on the detector upon which is superimposed the sharp cross sectional image of the desired focal plane within the object. Although any pattern of coordinated motion can be used, circular patterns are generally preferred because they are more easily produced.
U.S. Pat. No. 4,926,452 entitled "AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS", issued to Baker et al. describes a continuous circular scanned laminography system wherein the object remains stationary while the X-ray source and detector move in a coordinated circular pattern. The moving X-ray source comprises a microfocus X-ray tube wherein an electron beam is deflected in a circular scan pattern onto an anode target. The resulting motion of the X-ray source is synchronized with a rotating X-ray detector that converts the X-ray shadowgraph into an optical image so as to be viewed and integrated in a stationary video camera, thus forming a cross sectional image of the object. A computer system controls an automated positioning system that supports the item under inspection and moves successive areas of interest into view. In order to maintain high image quality, a computer system also controls the synchronization of the electron beam deflection and rotating optical system, making adjustments for inaccuracies of the mechanics of the system.
Laminographic cross sectional images may also be formed within the data memory of a computer by combining two or more individual images that were formed with coordinated positioning of two of the three main components comprising the laminography system, that is, a source, an object, and a detector. The images are combined within the computer memory such that any point in the object focal plane in one image is always combined with the same point in the object focal plane of another image, this other image consisting of a different angular view of the same object. If the individual views are taken with the detector describing a circular path, then the combined image formed from the individual images approaches the appearance of a continuous circular scanned image (as described in U.S. Pat. No. 4,926,452, discussed above) when the number of individual images is very large. Mathematically shifting the pixel combinations of the multiple individual images has the result of changing the location of the focal plane in the object. Thus, this method of generating a cross sectional image of an object has the advantage over moving and blurring methods, in that from one set of images, multiple laminographic cross sectional images of different focal planes may be formed. This technique has been called synthetic laminography, or computerized synthetic cross sectional imaging.
The laminography techniques described above are currently used in a wide range of applications including medical and industrial X-ray imaging. Laminography is particularly well suited for inspecting objects which comprise several layers having distinguishable features within each layer. However, some previous laminography systems which produce such cross sectional images typically experience shortcomings in resolution and/or speed of inspection, thus accounting for its rare implementation. These shortcomings are frequently due to the difficulties in achieving high speed coordinated motion of the source and detector to a degree of precision sufficient to produce a high resolution cross sectional image.
In a laminography system which views a fixed object and has a field of view which is smaller than the object being inspected, it may be necessary to move the object around within the field of view thus generating multiple laminographs which, when pieced together form an image of the entire object. This is frequently achieved by supporting the object on a mechanical handling system, such as an X, Y, Z positioning table. The table is then moved to bring the desired portions of the object into the field of view. Movement in the X and Y directions locates the area to be examined, while movement in the Z direction moves the object up and down to select the plane within the object where the cross sectional image is to be taken. While this method effectively enables various areas and planes of the object to be viewed, there are inherent limitations associated with the speed and accuracy of such mechanical motions. These constraints effectively act to increase the cycle time, thereby reducing the rates at which inspection can occur. Furthermore, these mechanical motions produce vibrations which tend to reduce the system resolution and accuracy.
U.S. Pat. No. 5,259,012 entitled "LAMINOGRAPHY SYSTEM AND METHOD WITH ELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCE", issued to Baker et al. describes a system which enables multiple locations within an object to be imaged without mechanical movement of the object. The object is interposed between a rotating X-ray source and a synchronized rotating detector. A focal plane within the object is imaged onto the detector so that a cross sectional image of the object is produced. The X-ray source is produced by deflecting an electron beam onto a target anode. The target anode emits X-ray radiation where the electrons are incident upon the target. The electron beam is produced by an electron gun which includes X and Y deflection coils for deflecting the electron beam in the X and Y directions. Deflection voltage signals are applied to the X and Y deflection coils and cause the X-ray source to rotate in a circular trace path. An additional DC voltage applied to the X or Y deflection coil will cause the circular path traced by the X-ray source to shift in the X or Y direction by a distance proportional to the magnitude of the DC voltage. This causes a different field of view, which is displaced in the X or Y direction from the previously imaged region, to be imaged. Changes in the radius of the X-ray source path result in a change in the Z level of the imaged focal plane. This system solves many of the problems of the early laminography systems in the generation of high resolution and high speed cross sectional images. This system is an improvement over that described in U.S. Pat. No. 4,926,452 in that it allows for the inspection of objects that are larger than the field of view by electronically generating cross sectional images off-axis to the rotation of the source and detector, thus eliminating a major source of mechanical motion. Additionally, the selection of the focal plane is accomplished by electronically sizing the diameter of the circular scan, thus eliminating the mechanical Z motion from the system described in U.S. Pat. No. 4,926,452. The method of generating cross sectional images described in U.S. Pat. No. 5,259,012 can theoretically go twice as fast as the system described in U.S. Pat. No. 4,926,452, since it does not have to wait for mechanical motion. It does have the same limitations as the system described in U.S. Pat. No. 4,926,452 as to source power and spot size limitations. Thus, total inspection speed is only a two to three times improvement, while adding considerable complexity in electronic circuitry and calibration efforts. While the system described in U.S. Pat. No. 5,259,012 does not require an X, Y, or Z table to position the object under inspection, it still needs a very complex and large X-ray tube to enable the system to work. The diameter of the X-ray tube must be slightly larger than the largest horizontal dimension of the object to be inspected with cross sectional imaging. Otherwise, the object, or the detector and X-ray tube, must be moved in the X direction and/or the Y direction, to inspect the entire object. Another disadvantage of this system is the requirement that the rotary detector imaging system relies on spinning a mechanical assembly at 600 or more revolutions per minute (RPM).
U.S. Pat. No. 5,020,086 entitled "MICROFOCUS X-RAY SYSTEM", issued to Peugeot discloses a system for tomosynthesis wherein an object is scanned by an X-ray beam from a circular position on a target resulting from the electron beam being scanned in a circle by appropriate control signals from a beam controller and applied to the deflection coils of a microfocus X-ray tube. Tomosynthesis is accomplished by the well known method of in-register combination of a series of digital X-ray images produced by X-ray beams emanating from different locations. This is achieved by positioning an X-ray source at multiple points on a circle around a central axis. This system eliminates some of the mechanical motion required by the system described in U.S. Pat. No. 4,926,452, in that the detector does not have to rotate. However, practical limitations of pixel size and resolution tend to limit the Peugeot system to inspection of items with small fields of view. Additionally, the system still requires an X, Y table to position the object under the field of view. The speed of a commercial prototype of this system is not significantly faster than the system described in U.S. Pat. No. 5,259,012, but may have a slightly lower cost of manufacture.
While there has been some well received commercial success of the system described in U.S. Pat. No. 4,926,452, and some commercial interest in both the system described in U.S. Pat. No. 5,020,086 and the system described in U.S. Pat. No. 5,259,012, industry still desires a cross sectional inspection system which operates at an even higher inspection speed while costing less than the existing industrial cross sectional inspection systems. If a new cross sectional imaging system could meet the demands of low cost and high performance, the commercial applications and usage would grow rapidly over the current technology and the benefit to the electronics industry for circuit board inspection would be greatly increased.
Accordingly, several objects and advantages of the present invention are that it provides an improved, lower cost, and simpler way to achieve high speed and high resolution cross sectional imaging for the inspection of electrical connections, than do previous systems.
It is one object of the present invention to eliminate the costly and complex scanned beam type X-ray tube used in U.S. Pat. Nos. 5,020,086 and 5,259,012, and replace the scanned beam X-ray tube with a standard low cost X-ray system.
It is another object of the present invention to eliminate the expensive X, Y positioning table (U.S. Pat. No. 5,020,086) or the X, Y, Z table (U.S. Pat. No. 5,259,012) with a low cost, single axis, highly reliable, continuous motion system.
It is another object of the present invention to replace the large diameter, expensive, and highly complex X-ray tube and system used in the U.S. Pat. No. 5,259,012 system, with a standard low cost X-ray system.
It is another object of the present invention to replace the complex rotating detector systems described in U.S. Pat. Nos. 4,926,452 and 5,259,012, and the large diameter and expensive vacuum tube detector disclosed in U.S. Pat. No. 5,020,086, with conventional, highly reliable, solid state, mass produced, low cost, high performance, linear line scan type detectors.